Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. A typical stimulation parameter set may include the electrodes that are sourcing (anodes) or returning (cathodes) the stimulation current at any given time, as well as the amplitude, duration, rate, and burst rate of the stimulation pulses. Significant to the present inventions described herein, a typical IPG may be manually inactivated by the patient by placing a magnet over the implanted IPG, which closes a reed switch contained within the IPG.
The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes using one or more current-controlled sources for providing stimulation pulses of a specified and known current (i.e., current regulated output pulses), or one or more voltage-controlled sources for providing stimulation pulses of a specified and known voltage (i.e., voltage regulated output pulses). The circuitry of the neurostimulator may also include voltage converters, power regulators, output coupling capacitors, and other elements as needed to produce constant voltage or constant current stimulus pulses.
The electrical stimulation energy may be delivered between a specified group of electrodes as monophasic electrical energy or multiphasic electrical energy. Monophasic electrical energy includes a series of pulses that are either all negative (cathodic), or alternatively all positive (anodic). Multiphasic electrical energy includes a series of pulses that alternate between positive and negative.
For example, multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) stimulation pulse (during a first phase) and an anodic (positive) charge recovery pulse (during a second phase) that is generated after the stimulation pulse to prevent direct current charge transfer through the tissue, thereby avoiding electrode degradation and cell trauma. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a stimulation period (the length of the stimulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the charge recovery pulse).
The second phase may have an active charge recovery pulse, wherein electrical current is actively conveyed through the electrode via current or voltage sources, and/or a passive charge recovery pulse, wherein electrical current is passively conveyed through the electrode via redistribution of the charge flowing from coupling capacitances present in the circuit, while the current or voltage sources are turned off. In the case of passive charge recovery, switches associated with the active electrodes are closed in order to passively convey the charge to AC ground.
Neurostimulation systems, which may not be limited to SCS used to treat chronic pain, are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic forces generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient.
In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the present of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m and much faster switching times (slew rates) of 150 mT/m/ms, which is comparable to stimulation therapy frequencies. Typical MRI scanners create gradient fields in the range of 100 Hz to 30 KHz, and radio frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner.
Despite the fact that an IPG implanted within a patient undergoing an MRI will be automatically deactivated (i.e., the magnetic field present in the MRI scanner will, via closing of the reed switch, automatically deactivate a IPG), the strength of the gradient magnetic field may be high enough to induce voltages (5-10 Volts depending on the orientation of the lead inside the body with respect to the MRI scanner) on to the stimulation lead(s), which in turn, are seen by the IPG electronics. If these induced voltages are higher than the voltage supply rails of the IPG electronics, there could exist paths within the IPG that could induce current through the electrodes on the stimulation lead(s), which in turn, could cause unwanted stimulation to the patient due to the similar frequency band, between the MRI-generated gradient field and intended stimulation energy frequencies for therapy, as well as potentially damaging the electronics within the IPG. To elaborate further, the gradient (magnetic) field may induce electrical energy within the wires of the stimulation lead(s), which may be conveyed into the circuitry of the IPG and then out to the electrodes of the stimulation leads via the passive charge recovery switches. For example, in a conventional neurostimulation system, an induced voltage at the connector of the IPG that is higher than IPG battery voltage (˜4-5V), may induce such unwanted stimulation currents. RF energy generated by the MRI scanner may induce electrical currents of even higher voltages within the IPG.
Typically, any electrical current blocking techniques, such as adding filters within the IPG, must be custom designed to suit a particular IPG. There, thus, remains a need to block electrical currents induced within the IPG via external means, such as MRI, with minimal modifications to the IPG.